1. Technical Field
The present subject matter relates to transmitting and receiving telemetry. More particularly, the subject matter relates to transmitting and receiving a discrete multi-tone modulated telemetry signal propagated in a fluid.
2. Background Information
Modern petroleum drilling and production operations demand a great quantity of information relating to parameters and conditions downhole. Such information typically includes characteristics of the earth formations traversed by the wellbore, along with data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole is referred to as “logging.”
Logging frequently is done during the drilling process, eliminating the necessity of removing or “tripping” the drilling assembly to insert a wireline logging tool to collect the data. Data collection during drilling also allows the driller to make accurate modifications or corrections as needed to optimize performance while minimizing down time. Designs for measuring conditions downhole, including the movement and location of the drilling assembly contemporaneously with the drilling of the well, have come to be known as “measurement-while-drilling” techniques, or “MWD”. Similar techniques, concentrating more on the measurement of formation parameters, commonly have been referred to as “logging while drilling” techniques, or “LWD”. While distinctions between MWD and LWD may exist, the terms MWD and LWD often are used interchangeably. For purposes of this disclosure, the term LWD will be used with the understanding that this term encompasses both the collection of formation parameters and the collection of information relating to the movement and position of the drilling assembly.
Sensors or transducers are located within “tools” at the lower end of the drillstring in LWD systems. In particular, sensors employed in LWD applications are positioned in a cylindrical drill collar that is positioned close to the drill bit. While drilling is in progress these sensors continuously or intermittently monitor predetermined drilling parameters and formation data, and the tools transmit the information to a surface detector by some form of telemetry. There are a number of communication schemes in the related art that transmit information regarding downhole parameters to the surface, such as mud pulse telemetry systems.
Mud pulse telemetry systems create pressure pulses in the drilling fluid within the drillstring. The information that is acquired by the downhole sensors is transmitted by suitably timing pressure pulses in the drilling fluid. The information is received and decoded by a pressure transducer and computer at the surface. Data transmission rates achievable through mud pulse systems have generally been limited to around 1 Hz, restricting the amount of information that can be transmitted real-time as drilling is taking place. Accordingly, a downhole telemetry system capable of higher data rates is desirable.